1. Field of the Invention
The present invention relates to a belt type continuously variable transmission used as a continuously variable transmission for a vehicle, for example, and more particularly to a hydraulic actuator for a belt type continuously variable transmission in which a pulley groove width is varied by moving a movable sheave that is disposed opposite to a fixed sheave formed integrally with a pulley shaft.
2. Description of the Related Art
A belt type continuously variable transmission includes: a primary shaft and a secondary shaft disposed in parallel; a primary pulley having a fixed sheave formed integrally with the primary shaft and a movable sheave that is disposed opposite to the fixed sheave and is capable of moving in an axial direction; a secondary pulley having a fixed sheave formed integrally with the secondary shaft and a movable sheave that is disposed opposite to the fixed sheave and is capable of moving in the axial direction; a driving belt wound around the pulleys; and an actuator for moving the movable pulleys in the axial direction. The belt type continuously variable transmission performs speed shifts continuously by employing the actuator to move the movable sheaves, thereby varying a pulley groove width such that a ratio of an effective winding diameter of the driving belt relative to the respective pulleys is varied.
Incidentally, a hydraulic actuator is used as an actuator for moving the movable sheave of the pulley in the axial direction, for example, Japanese Unexamined Patent Application No. H2-62449 and Japanese Unexamined Patent Application No. H3-33542 have proposed a hydraulic actuator known as a tandem type hydraulic actuator for securing an increase in a pushing force applied to the movable sheave without an increase in outer diameter.
The tandem type hydraulic actuator disclosed in Japanese Unexamined Patent Application No. H2-62449 will now be described with reference to FIG. 7, which is a sectional view showing the main parts of a belt type continuously variable transmission.
A primary pulley 102 includes a fixed sheave 103 formed integrally with a primary shaft 101 and a movable sheave 104 that is disposed opposite to the fixed sheave 103 and is capable of moving in an axial direction, and a hydraulic actuator 105 for moving the movable sheave 104 in the axial direction is disposed between a back surface of the movable sheave 104 and the primary shaft 101.
The hydraulic actuator 105 includes a cylindrical first cylinder 106 that projects from an outer periphery of the movable sheave 104 toward the back surface side, a fixed wall 107 having an outer periphery that is fitted slidably to an inner peripheral surface of the first cylinder 106 and formed with a tubular portion 107a, an inner periphery of which is fixed to the primary shaft 101 such that a first hydraulic chamber A is formed between the fixed wall 107 and the movable sheave 104, a drum 108 having a cylindrical second cylinder 108a that covers an outer side of the first cylinder 106 so as to overlap the first cylinder 106 in an axial direction, an inner periphery of which is fixed to the primary shaft 101, and a cylindrical plunger 109 having an inner peripheral surface that is fitted slidably to an outer peripheral surface of the tubular portion 107a and an outer periphery that is fitted slidably between the inner periphery of the second cylinder 108a and the first cylinder 106 such that a second hydraulic chamber B is formed between the plunger 109 and the drum 108.
An oil pressure supply passage 101a is formed in the interior of the primary shaft 101, and working oil pressure introduced into the oil pressure supply passage 101a is supplied to the first hydraulic chamber A via a passage 101b formed in the primary shaft 101 in a diametrical direction and supplied to the second hydraulic chamber B via an oil hole 107b formed in an inner peripheral portion of the fixed wall 107. The movable sheave 104 is pushed in the direction of the fixed sheave 103 directly by the working oil pressure of the first hydraulic chamber A, and the plunger 109 is moved by the working oil pressure of the second hydraulic chamber B. The moving force of the plunger 109 is transmitted to the movable sheave 104 to move the movable sheave 104 in the same direction together with the pushing force of the first hydraulic chamber A. As a result, a pulley groove width of the primary pulley 102 is narrowed. Accordingly, an effective winding diameter of a driving belt 110 wound around the primary pulley 102 increases, leading to an increase in a gear ratio that is transmitted to a secondary pulley, not shown in the drawing.
Meanwhile, when the working oil pressure is discharged from the first hydraulic chamber A and the second hydraulic chamber B, the movable sheave 104 and the plunger 109 are moved in a direction heading away from the fixed sheave 103 by tension in the driving belt 110. As a result, the effective winding diameter of the driving belt 110 wound around the primary pulley 102 decreases, decreasing the gear ratio relative to the secondary pulley.
The tandem type hydraulic actuator disclosed in Japanese Unexamined Patent Application No. H3-33542 will now be described with reference to FIG. 8, which is a sectional view showing the main parts of a belt type continuously variable transmission.
A primary pulley 112 includes a fixed sheave 113 formed integrally with a primary shaft 111 and a movable sheave 114 that is disposed opposite to the fixed sheave 113 and is capable of moving in an axial direction, and a hydraulic actuator 115 for moving the movable sheave 114 in the axial direction is disposed between a back surface of the movable sheave 114 and the primary shaft 111.
The hydraulic actuator 115 includes a cylindrical first cylinder 116 that projects from an outer periphery of the movable sheave 114 toward the back surface side, a drum 117 that is fixed to the primary shaft 111 and includes, on its outer periphery, a second cylinder 117a that covers an outer side of the first cylinder 116 so as to overlap the first cylinder 116 in the axial direction, a first plunger 118 that includes a tubular portion 118a formed in a tubular shape, an inner end of which is press-fitted fixedly into a press fitting groove 117b formed in a ring shape in a side face of the drum 117 and an outer end of which is fitted slidably to an inner peripheral surface of the first cylinder 116, whereby a first hydraulic chamber A is formed between the first plunger 118 and the movable sheave 114, and a second plunger 119 having an outer periphery that is fitted slidably to the inner peripheral surface of the first cylinder 116 and an inner periphery that is fitted slidably to the tubular portion 118a of the first plunger 118, whereby a second hydraulic chamber B is formed between the second plunger 119 and the drum 117.
An oil pressure supply passage 111a is formed in the interior of the primary shaft 111, and working oil pressure from the oil pressure supply passage 111a is supplied to the first hydraulic chamber A and the second hydraulic chamber B. The movable sheave 114 is pushed in the direction of the fixed sheave 113 directly by the working oil pressure of the first hydraulic chamber A, and the second plunger 119 is moved by the working oil pressure of the second hydraulic chamber B. The moving force of the plunger 119 is transmitted to the movable sheave 114 so as to move the movable sheave 114 in the same direction together with the pushing force of the first hydraulic chamber A. As a result, a pulley groove width of the primary pulley 112 is narrowed. Accordingly, the effective winding diameter of a driving belt 120 wound around the primary pulley 112 increases, leading to an increase in the gear ratio that is transmitted to a secondary pulley, not shown in the drawing.
Meanwhile, when the working oil pressure is discharged from the first hydraulic chamber A and the second hydraulic chamber B, the movable sheave 114 and the second plunger 119 are moved in a direction heading away from the fixed sheave 113 by tension in the driving belt 120. As a result, the effective winding diameter of the driving belt 120 decreases, decreasing the gear ratio relative to the secondary pulley.
According to the hydraulic actuators of the belt type continuously variable transmissions described in Japanese Unexamined Patent Application No. H2-62449 and Japanese Unexamined Patent Application No. H3-33542, the first hydraulic chamber and the second hydraulic chamber are disposed coaxially with the primary shaft, and therefore the pushing force applied to the movable sheave can be increased without increasing in outer diameter.
However, in Japanese Unexamined Patent Application No. H2-62449, the fixed wall 107 that forms the first hydraulic chamber A and supports the plunger 109 slidably is fixed integrally to the primary shaft 101, and therefore the oil passage 107b for supplying the working oil pressure from the oil pressure supply passage 101a of the primary shaft 101 to the second hydraulic chamber B must be drilled into the fixed wall 107. As a result, the constitution of the fixed wall 107 becomes complicated, and the labor involved in the drilling operation leads to an increase in manufacturing cost.
In Japanese Unexamined Patent Application No. H3-33542, meanwhile, the first hydraulic chamber A is formed between the first plunger 118 and the movable sheave 114 by fixedly press-fitting the tubular portion 118a of the first plunger 118 into the press fitting groove 117b formed in a ring shape in the side face of the drum 117, and the second hydraulic chamber B is formed between the drum 117 and the second plunger 119 by slidably fitting the inner periphery of the second plunger 119 into the tubular portion 118a. 
Hence, the working oil pressure of the first hydraulic chamber A acts on the first plunger 118, and due to the need to secure a smooth operation in the second plunger 119, on which the working oil pressure of the second hydraulic chamber B acts, sufficient joining rigidity is required in a joint portion between the cylindrical portion 118a of the first plunger 118 and the drum 117, or in other words the press fitting groove 117b and the cylindrical portion 118a. Moreover, an oil passage for supplying working oil pressure to the second hydraulic chamber B must be formed on the end portion of the cylindrical portion 118a of the first plunger 118 and in an extremely restricted site that does not affect the fitting with the press fitting groove 117b, and therefore the constitution of the first plunger 118 becomes complicated and formation of the oil passage is troublesome, leading to an increase in manufacturing cost.